The field of the invention is corona treater stations, and particularly, the construction of electrodes for such stations.
Corona treater stations for sheets, or webs of plastic materials take many forms. Typically, the material to be treated is fed through a treatment zone in which one surface of the material is bombarded with ions produced by a high voltage alternating electric field. The material is supported in the treatment zone by a conductive roller which also serves as one electrode for the treater. A second, active electrode is supported in the treatment zone and is spaced from the conductive roller equidistantly along its length. For many years the active electrode has taken the form of metal segments which are pivotally attached to a bar, or beam, and which can be selectively swung away from the roller to remove them from the treatment zone. In this manner, the width of the treatment zone can be adjusted to accommodate webs of different widths.
One difficulty with segmented electrodes is that they produce an uneven treatment. This may be caused either by an uneven spacing of the segments from the roller electrode or by changes in the electric field strength at the junction of adjacent segments.
Treater station reliability has been significantly increased recently by employing an elongated active electrode having a coating, or jacket, made of a heat resistant insulating material. Such a structure is disclosed in co-pending U.S. patent application Ser. No. 237,964 filed on Feb. 25, 1981, where the active electrode is constructed from a hollow quartz rod which is filled with a conductive material and which extends the entire length of the roller electrode. While such constructions provide very uniform treatment and do not require an insulating layer on the roller electrode, they are difficult to manufacture. It is difficult to form long tubes of quartz of ceramic materials and it is difficult to fill them with conductive material. Also, the differences in the thermal coefficients of expansion of the insulating rod and its supporting structure make it difficult to mount the rods.
A number of attempts have been made recently to provide a segmented active electrode in which each segment is covered with an insulating jacket. Characteristically, such structures have employed a metal bar or beam which extends along the length of the roller electrode and which supports a series of metal fingers that each extend from the beam toward the roller electrode. In this respect they are very similar to the well known segmented metal electrodes in that the separate metal fingers can be removed or swung away to adjust the width of the treatment zone. The difference is that a jacket of insulating material is slipped over the end of each metal finger to allow bare roll treating. The uneven treatment produced by segmented active electrodes is greatly amplified when the segments are insulated in this manner, and such constructions have not thus far been commercially successful.